Temperature detector

ABSTRACT

Zero-cross points of an ac voltage waveform (v) applied to a heat-sensitive wire (1) and an ac current waveform (i) flowing into the heat-sensitive wire (1) are detected by a zero voltage detection unit (3) and a zero current detection unit (4), and the interval between the zero-cross points of voltage and current is measured by a measuring unit (7). The measured interval corresponds to the phase angle (φ) between the voltage and current waveforms, and the phase angle (φ) depends on the specific resistance (ρ) and permitivity (ε) of the heat-sensitive wire (1). This relation is utilized for temperature measurement. The relation between the phase angle (φ) and the specific resistance (ρ) and permitivity (ε) is independent of the size of the heat-sensitive wire. Therefore, the temperature can be correctly measured even when the wire is made of a material which is not rigid enough to keep its size constant.

TECHNICAL FIELD

The present invention relates to a temperature detector using aheat-sensitive wire as a temperature measuring element.

DESCRIPTION OF THE BACKGROUND ART

A heat-sensitive wire is obtained such that a thermistor made of apolymer compound is buried between coaxial internal and externalconductors, and they are drawn to have a linear shape. In thisheat-sensitive wire, an impedance between the conductors is variedaccording to a temperature. The impedance is low at a high temperature,and the impedance is high at a low temperature. A temperature detectoris formed using the above characteristics of the heat-sensitive wire.

The impedance of the heat-sensitive wire is mainly constituted by aresistance component obtained by moving ions contained in a polymercompound and an electrostatic capacitance between the conductors. Whenthe heat-sensitive wire is used as a temperature measuring element, anerror occurs in a measurement value due to localization of ions orcharge accuraulation in a capacitor, and the degradation of theheat-sensitive wire is accelerated. For this reasons, an ac voltagehaving symmetrical positive and negative components must be applied tothe heat-sensitive wire, and a commercial power source having a sinewave is conventionally used to measure the impedance. For example, thefollowing method is described in Japanese Patent Laid-Open No. 59-44803.That is, an ac voltage is applied to a temperature detecting wire, andtemperature detection is performed by the magnitude of the impedance ofthe temperature detecting wire to control a heating power.

An impedance z of the heat-sensitive wire can be represented by avector. When the above arrangement is regarded as a series equivalentcircuit, assuming that the angular velocity of a power source frequencyis represented by ω; a resistance component, R; and an electrostaticcapacitance, C, the impedance Z can be represented by the followingequation: ##EQU1## In this equation, R and C of the right-hand side arechanged in accordance with a temperature. As a result, Z of theleft-hand side has a value corresponding to the temperature.

Although the heat-sensitive wire has also an inductance component as amatter of course, when the heat-sensitive wire has a practical length(several meters to several tens of meters) , the magnitude of theinductance component is smaller than that of the resistor component orthe electrostatic capacitance enough to be negligible. Therefore, theinductance component is omitted in equation (1).

An absolute value |Z| of the impedance Z is calculated by equation (1)as follows: ##EQU2## In a conventional temperature detector of thistype, a temperature is detected by the following means. That is, theimpedance of a heat-sensitive wire is calculated as the absolute value|Z|, and an absolute value |I| of a current I obtained by applying avoltage E to the heat-sensitive wire is calculated. A voltage obtainedby causing the current I to flow into the heat-sensitive wire directlyor through a resistor having a known resistance is converted into a dcvoltage, and the current or voltage is compared with a dc referencecurrent or voltage obtained independently of the measured voltage orcurrent.

The impedance of the heat-sensitive wire will be described below indetail.

The section of the heat-sensitive wire is shown in FIG. 1. Assume thatthe length of a heat-sensitive wire 1 is represented by L; the outerradius of an internal conductor 101, a; the inner radius of an externalconductor 102, b,; the specific resistance of a polymer compound 103, ρ;and a permitivity, ε, and assume that a terminal effect is neglected. Inthis case, a resistance component R and an electrostatic capacitance Care obtained by the following equations: ##EQU3##

On the other hand, when the heat-sensitive wire is applied to anelectric blanket or the like, the flexibility of it is an importantproperty. The internal conductor 101 is obtained by winding a thin wireor a conductive ribbon around a core made of synthetic fibers, and theexternal conductor 102 has the same structure as that of the internalconductor 101. Therefore, the outer radius a and the inner radius b ofthe conductors cannot be easily processed with high dimensionalprecision in the manufacturing process, and the impedance Z and itsabsolute value |Z| represented by equations (1) and (2) obtained asresults of the resistance component R and the electrostatic capacitanceC represented by equations (3) and (4) are considerably varied. Forexample, when the length L is 5 m, a precision of ±30% is obtained, andeven when the length is 10 m, only a precision of ±20% is obtained. Forthis reason, in practical use, after the heat sensitive wire is cut tohave a predetermined length, the cut wire is discriminated, or the cutwire is connected to a detector to determine whether the cut wire isproper in practice.

In Japanese Patent Laid-Open No. 60-125533, the following method isdescribed. That is, a temperature is measured by a relationship betweenthe magnitude of a current supplied to a power cable and the phasedifference between a voltage applied to the power cable and the current.When this measuring method is applied to a heat-sensitive wire, sincethe phase difference between the voltage and the current is defined bythe specific resistance permitivity of a thermistor materialindependently of the size of the heat-sensitive wire, temperaturedetection can be accurately performed regardless of the dimensionalprecision of the heat-sensitive wire. A measuring method in which thephase difference between the voltage and the current is detected bydetecting an interval between zero-cross points of each waveform isdisclosed, in e.g., Japanese Patent Laid-Open No. 51-3275.

However, extensive studies of the present inventor found that it wasvery important to apply an ac voltage having symmetrical positive andnegative components to the heat-sensitive wire, and that even when aslight unbalanced component was present, a measurement error occurred,or the thermistor material of the heat-sensitive wire was degraded toincrease the measurement error. In relation to this point, it was foundthat in the well-known method (for example, Japanese Patent Laid-OpenNo. 54-136877) in which the phases of an ac waveform were detected bydetecting zero-cross points of the waveform, the occurrence of theunbalanced component could not be completely eliminated.

SUMMARY OF THE INVENTION

It is a principal of the present invention to provide an apparatuscapable of solving the above problems and performing correct temperaturedetection regardless of the dimensional precision of a heat-sensitivewire.

It is an object of the present invention to prevent occurrence of anunbalanced component in an ac waveform applied to a heat-sensitive wireto decrease a measurement error so as to prevent the degradation of theheat-sensitive wire.

It is another object of the present invention to obtain a highlyaccurate temperature measurement value using a flexible heat-sensitivewire which is susceptible to variations in dimensional precision.

It is still another object of the present invention to achieve theprincipal by digital processing.

It is still another object of the present invention to achieve theprincipal by analog processing.

It is still another object of the present invention to provide ameasuring circuit capable of easily performing especially digitalprocessing.

It is still another object of the present invention to obtain atemperature measurement value when temperature measurement is performedby an ac voltage without any influence of the frequency of the acvoltage.

According to the present invention, there is provided a temperaturedetector comprising zero voltage detecting means, connected in parallelto the heat-sensitive wire, for detecting zero-cross points of an acvoltage waveform applied to a heat-sensitive wire, zero currentdetecting means, connected in series with the heat-sensitive wire, fordetecting zero-cross points of an ac current waveform flowing into theheat-sensitive wire, and interval measuring means for receiving outputsfrom the zero voltage detecting means and the zero current detectingmeans to measure an interval between the zero-cross points of thevoltage waveform and the current waveform, wherein the zero currentdetecting means has at least the same impedances of the positive andnegative components of the ac waveform to prevent generation of anunbalanced component in an ac current waveform flowing across both theconductors, and a temperature measurement value is obtained from anoutput from the interval measuring means.

That is, when equation (1) is rewritten using equation (2), thefollowing equation can be obtained: ##EQU4##

Although the derivation process for these equations will be omitted,equations (5) and (6) are obviously obtained by the following equations:##EQU5##

In addition, substitutions of equations (3) and (4) into equation (6)yield the following equation: ##EQU6##

In this case, the specific resistance ρ and the permitivity ε of thepolymer compound 103 are constants respectively determined by thephysical properties of the polymer compound itself regardless of thesize of the heat-sensitive wire, and the angular velocity ω is aconstant determined by the frequency of a power source. Therefore, φrepresented in equation (7) has a numerical value which is independentof the size of the heat-sensitive wire, and the numerical valuecorresponds to a temperature.

On the other hand, φ corresponds to a voltage vector E applied to theheat-sensitive wire and the phase angle of a current vector I flowinginto the heat-sensitive wire at the time of application of the voltageE. For this reason, when the phase angle φ is measured, highly accuratetemperature detection can be performed regardless of the dimensionalprecision of the heat-sensitive wire.

The phase angle φ can be measured with high accuracy by measuring aninterval between the zero-cross points of a current waveform and avoltage waveform. This interval measurement is completed preferablybased on digital processing performed by counting clock pulses. Inaddition, an interval measurement value can be output as an analogamount.

According to one aspect of the present invention, a heat-sensitive wireis obtained as follows. That is , a thermistor made of a polymercompound is buried between coaxial internal and external conductors, andthey are drawn to have a linear shape. Since a specific resistance ρ anda permitivity ε of the polymer compound constituting the thermistor arechanged with a temperature, the temperature can be calculated from aphase angle φ on the basis of equation (7).

According to a preferred aspect of the present invention, a means formeasuring the angular frequency ω in equation (7) is provided to removethe influence of a change in measurement frequency.

According to another aspect of the present invention, a pulse generatingcircuit is used as a means for detecting zero current and voltagewaveforms, thereby easily performing digital processing using, e.g., amicroprocessor.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus, are notlimitative of the present invention and wherein:

FIG. 1 is a sectional vi showing a heat-sensitive wire;

FIG. 2 is a circuit diagram showing a temperature detector according toan embodiment of the present invention;

FIG. 3 is a waveform chart of an operation in the embodiment shown inFIG. 2;

FIGS. 4 and 5 are circuit diagrams showing main parts of a temperaturedetector according to another embodiment of the present invention; and

FIG. 6 is a circuit diagram showing a main part of a temperaturedetector when an angular frequency is added to measurement factors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described below with reference to theaccompanying drawings.

FIG. 2 shows a temperature detector according to an embodiment of thepresent invention. In this temperature detector, an ac power source 2 isapplied to a heat-sensitive wire 1 through a protecting resistor 5,zero-cross points of an instantaneous voltage v applied to theheat-sensitive wire 1 are detected by a zero voltage detection unit 3,and zero-cross points of a current j, flowing into the heat-sensitivewire 1 are detected by a zero current detection unit 4. Both intervalsbetween the zero-cross points are measured by an interval measuring unit7, and an output unit 8 outputs a temperature measurement value Tobtained by converting the measurement value measured by the measuringunit 7 into a signal having a form suitable for an equipment connectedto the output of the output unit 8.

The zero voltage detection unit 3 has a detection transistor 301, andthe base of the transistor 301 is connected to the power source terminalof the heat-sensitive wire 1 through a resistor 303 and to a ground linethrough a diode 302. When the voltage v (FIG. 3(a) of the ac powersource 2 is in a positive half-wave integral of the waveform period, abase current flows into the transistor 301 through the resistor 303 toturn on the transistor 3O1. When the voltage of the ac power source 2 isin a negative half-wave interval of the waveform period, the diode 302is turned on, and the transistor 301 is turned off. Therefore, as shownin FIG. 3(b), a detection pulse B which is set to "0" level in apositive half-wave interval of the voltage v of the power source 2 andset to "1" level in a negative half-wave interval thereof can beobtained from the collector of the transistor 301.

The zero current detection unit 4 has the same arrangement as that ofthe zero voltage detection unit 3. The zero current detection unit 4includes a detection transistor 401 which has the base connected to theground terminal of the heat-sensitive wire 1 and which is clamped at analmost ground potential by a diode 402. This transistor 401 outputs adetection pulse C from its collector in response to the ac current i,(FIG. 3(a)) flowing into the heat-sensitive wire 1. As shown in FIG. 3(c) , the detection pulse C is set to "0" level in a positive half-waveinterval of the period of the current i set to "1" level in a negativehalf-wave interval of the period of the current i.

The voltage of the commercial ac power source 2 is set to an effectivevalue of several 10 to 100 V, and each of the threshold voltages of thetransistors 301 and 401 and the diodes 302 and 402 is set at a dcvoltage of 0.7 V or less. Therefore, when these threshold voltages areneglected, the zero-cross points of the instantaneous voltage v appliedto the heat-sensitive wire 1 and the instantaneous current i flowinginto the heat-sensitive wire 1 can be detected as leading and trailingedges of the collector voltages of the transistors 301 and 401.

The instantaneous voltage v and the instantaneous current j, areillustrated by the operating waveforms shown in FIG. 3. That is, in FIG.3 (a) , time is plotted in the abscissa, and the instantaneous voltage vapplied to the heat-sensitive wire 1 and the instantaneous current iflowing into the heat-sensitive wire 1 are indicated. The collectorvoltage of the transistor 301 changed to correspond to each time isshown in FIG. 3(b), and the collector voltage of the transistor 401 isshown in FIG. 3(c). Assuming that these voltages are represented bylogical values B and C, respectively, the phase angle φ corresponds toan interval in which B·C=1 or B·C=1 is established. In both theintervals, the diode 302 having a polarity opposite to that of thetransistor 301 is connected between the base and emitter of thetransistor 301, and the diode 402 having a polarity opposite to that ofthe transistor 401 is connected between the base and emitter of thetransistor 401, thereby obtaining the instantaneous voltage v and theinstantaneous current i each having symmetrical positive and negativecomponents. In addition, each of all intervals between the zero-crosspoints of the instantaneous voltage v and the zero-cross points of theinstantaneous current i corresponds to the phase angle φ at a ratio of1:1. For this reason, when convenient zero-cross points are selected,and an interval between the zero-cross points is measured, the phaseangle φ can be obtained.

The length of the interval can be variously measured by well-knowncircuit techniques. In an embodiment in FIG. 2, a NOT element 701 and anAND element 702 cause a counter 704 to count clock pulses CK generatedby an oscillator 703 in only an interval in which B·C=1 is established,and the counter 704 is reset in an interval of C=1 or at the leadingedge of the interval. With the above arrangement, the length of theinterval, i.e., the phase angle φ, in which B·C=1 is established can becontinuously measured every hertz, thereby obtaining satisfactoryresponse in consideration of the response of the heat-sensitive wire 1.

In another embodiment shown in FIG. 4, a NOT element 711 and an ANDelement 712 cause the AND element 712 to output a voltage in a period inwhich B·C=1 is established The output voltage gas a pulse widthcorresponding to an interval between the zero-cross points and isintegrated by an integrating circuit 713 to obtain a voltagecorresponding to a size of a phase angle φ. This voltage is measured bya voltage measuring unit 715 to achieve an object of the presentinvention. Charges accumulated in the integrating circuit 713 aredischarged by a transistor 714 in a period of C=1, such that theintegrating circuit 713 prepares for the next measurement.

In a digital processing circuit such as a counter, a signal is generallyprocessed as a pulse shape rather than as a change in level. When eachof the signals B and C passes through a differential circuit, and thenthe signals are used such that the polarities of the signals arematched, the signals can be suitable for the above object.

As shown in FIG. 5, a zero voltage detection unit 3 and a zero currentdetection unit 4 may have an arrangement in which a voltage and acurrent are rectified by a diode bridge 412 to be supplied to atransistor 411. In this case, since the collector voltage of thetransistor 411 is obtained as a pulse waveform at a zero-cross point, apulse processing technique is easily applied to the collector voltage.However, a power source 6 for the transistor 411 and the ground line ofan ac power source 2 are not easily connected to each other.

In either case, since the heat-sensitive wire is an element having aconsiderably high impedance, a high input-impedance element such as afield effect transistor is preferably used as a transistor.

In addition to the above methods, as a method of measuring the phaseangle φ, it may be suitable that a proper level point of aninstantaneous voltage or an instantaneous current, e.g., a maximum valuepoint, is measured. However, since the heat-sensitive wire has a largenumber of nonlinear factors, a current flowing into the heat-sensitivewire is distorted, and the shape factor of the current is changed inaccordance with a change in temperature. For this reason, when the phaseangle is measured at points other than zero-cross points, an increase inerror of the phase angle must be taken into consideration. This currentdistortion has been neglected in the above description for the sake ofdescriptive convenience. However, it should be understood that thecurrent distortion has been described with reference to an equivalentsine wave.

As is apparent from equation (7), when the frequency of a power sourceis changed, the phase angle φ is changed as a matter of course.Therefore, although the above measurement value is changed, as isillustrated in FIG. 6, the measurement value can be corrected byindependently measuring the length of an interval in which the signal Bor C is set to "1" level or "0" level. In FIG. 6, an AND element 710output oscillator-clock pulses from oscillator 703 of in an interval inwhich a zero voltage detection pulse B is set at "1" level, and theclock pulses are counted by a counter 716, thereby obtaining datacorresponding to an angular frequency ω. This data is supplied togetherwith data of a phase angle φ which represents a measurement value of theinterval of the zero-cross points as described previously, to an outputunit 8 constituted by, e. g. , a microprocessor or the like. The phaseangle φ is output from the counter 704 shown in FIG. 1. Therefore, atemperature T (corresponding to the phase angle φ) can be obtained onthe basis of equation (7).

As described above, according to the present invention, zero-crosspoints of the instantaneous voltage v applied to the heat-sensitive wire1 and the instantaneous current i flowing into the heat-sensitive wire 1are detected by the zero voltage detection unit 3 and the zero currentdetection unit 4, respectively, intervals between the zero-cross pointsare measured by a zero-cross point interval measuring unit 7 to obtain ameasurement value corresponding to the phase angle φ. By using thismethod, a temperature detector capable of detecting an accuratetemperature regardless of the dimensional precision of theheat-sensitive wire can be obtained.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded s adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A temperature detector for detecting atemperature by applying an ac power source across coaxial internal andexternal conductors of a heat-sensitive wire, comprising:zero voltagedetecting means, parallelly connected to said heat-sensitive wire, fordetecting zero-cross points of an ac voltage waveform applied acrossboth of said coaxial internal and external conductors; zero currentdetecting means, connected in series with said heat-sensitive wire, fordetecting zero-cross points of an ac current waveform flowing acrossboth of said coaxial internal and external conductors; and intervalmeasuring means for receiving outputs from said zero voltage detectingmeans and said zero current detecting means to measure an intervalbetween the zero-cross points of the ac voltage waveform and the accurrent waveform, said zero current detecting means having at least thesame impedances of positive and negative components of the ac currentwaveform to prevent generation of an unbalanced component in the accurrent waveform flowing across both of said coaxial internal andexternal conductors, a temperature measurement value being obtained froman output of said interval measuring means.
 2. The temperature detectoraccording to claim 1, wherein a thermistor is buried between saidcoaxial internal and external conductors.
 3. The temperature detectoraccording to claim 2, wherein said thermistor is made of a polymercompound and said coaxial internal and external conductors are flexible.4. The temperature detector according to claim 1, wherein said zerovoltage detecting means and said zero current detecting meansrespectively apply an ac voltage waveform to said heat-sensitive wireand supply an ac current waveform which flows into said heat-sensitivewire as base inputs of transistors, first and second diodes each havinga polarity opposite to that of a corresponding transistor respectivelybeing connected between a base and an emitter of each of saidtransistors, and ON/OFF threshold levels of said transistors are set toan approximately zero voltage level and an approximately zero currentlevel, respectively.
 5. The temperature detector according to claim 1,wherein said interval measuring means comprises counter means forcounting clock pulses in an interval between the zero-cross points. 6.The temperature detector according to claim 1, wherein said intervalmeasuring means comprises an integrator for integrating a pulse having awidth corresponding to the interval between the zero-cross points toform a voltage signal corresponding to a temperature.
 7. The temperaturedetector according to claim 1, wherein said zero voltage detecting meansand said zero current detecting means comprise:a rectifier forrectifying the ac voltage waveform applied to said heat-sensitive wireand the ac current waveform flowing into said heat-sensitive wire; and adetecting transistor having ON/OFF threshold levels which are set to azero voltage level and a zero current level.
 8. The temperature detectoraccording to claim 1, wherein said interval measuring meanscomprises:measurement means for generating a measurement valuecorresponding to of the ac voltage waveform applied to saidheat-sensitive wire and the ac current waveform flowing into saidheat-sensitive wire; and calculating means for calculating a temperaturemeasurement value on the basis of measurement values of the frequenciesand a measurement value of the interval of the zero-cross points.
 9. Thetemperature detector according to claim 1, wherein said zero voltagedetecting means comprises:a first transistor having a base coupled tothe ac voltage waveform applied to said heat-sensitive wire, an ON/OFFthreshold of said first transistor being set to an approximately zerovoltage level; and a first diode, of polarity opposite said firsttransistor, coupled between the base and an emitter of said firsttransistor, said zero current detecting means comprises a secondtransistor having a base coupled to the ac current waveform flowing intosaid heat-sensitive wire, an ON/OFF threshold of said second transistorbeing set to an approximately zero current level; and a second diode, ofpolarity opposite said second transistor, coupled between the base andan emitter of said second transistor.